Switch mode power supply for a gas discharge lamp

ABSTRACT

A switch mode power supply for igniting and operating a high-pressure gas discharge lamp includes an ignition sub-circuit which is coupled via a transformer to the circuit. The ignition sub-circuit has at least an ignition capacitor, and a switching element for discharging the capacitor at a desired moment in time having a control electrode. The control electrode of the switching element is connected to the commutator of the lamp circuit via a control sub-circuit having substantially passive elements.

This application claims priority to International Application No.PCT/IB02/02102 published on Jun. 5, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to a switch mode power supply for ignitingand operating a high-pressure gas discharge lamp, wherein the switchmode power supply is provided with an ignition sub-circuit forgenerating voltage pulses for igniting the lamp.

WO-0 169 985 discloses a lamp operating circuit of a known type,comprising an ignition circuit which comprises a switch and a pulsecapacitor which is connected via an ohmic resistor to a buffercapacitor, wherein pulses for igniting the lamp can be generated at afrequency of at least 100 Hz. A drawback of this known switch mode powersupply is that the pulse generation is uncoordinated, independently ofthe current available for the lamp, so the chance of ignition isrelatively small. As a result a certain quantity of wolfram evaporatesfrom the electrodes during startup of the lamp, whereby the light outputof the lamp deteriorates and the lifespan thereof is shortened. Inaddition, a separate control unit is required which is complex andcomprises a large number of components.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention attempts to obviate the above stated problems, andprovides for this purpose a switch mode power supply for igniting andoperating a high-pressure gas discharge lamp, having a lamp circuit andcomprising an ignition sub-circuit, which is coupled via a transformerto the lamp circuit, wherein the ignition sub-circuit comprises:

at least an ignition capacitor;

a switching element for discharging the capacitor at a desired moment intime having a control electrode;

wherein the control electrode of the switching element is connected tothe commutator of the lamp circuit via a control sub-circuit whichcomprises substantially passive elements.

Such a switch mode power supply comprises a simple ignition circuit witha smaller number of components. A better ignition in cold state is alsoobtained, whereby the light output of the lamp is improved and thelifespan extended.

In a further preferred embodiment the switching element is a so-calledTRIAC. Such a switching element is generally available on the market andis simple to control.

In a further preferred embodiment the capacitor and the switchingelement are coupled via the primary winding of the transformer to thelamp circuit and form therewith a resonant circuit such that the currentthrough the ignition sub-circuit generates a voltage pulse of a desiredlevel over the secondary winding of the transformer.

In a further preferred embodiment the switch mode power supply comprisesat least one buffer capacitor for storing a voltage, and the ignitionsub-circuit comprises at least one resistor which is connected on oneside to the buffer capacitor and on the other side to the at least onecapacitor of the ignition sub-circuit. The energy for the voltage pulsesis thus drawn directly from the buffer capacitor so that fewercomponents are required and the switch mode power supply becomes morecompact.

In a further preferred embodiment the switch mode power supply comprisesa so-called Half Bridge Commutating Forward circuit, so that the switchmode power supply becomes more compact and fewer components arerequired.

In a further preferred embodiment the switch mode power supply comprisesat least one DIAC, for instance with a breakover voltage of 30 V, formaking the switch element conductive at a predetermined moment.

In a further preferred embodiment the switch mode power supply comprisesa SIDAC so that the switch element does not remain conductive after theat least one capacitor has been discharged.

According to a further aspect, the present invention provides a methodfor igniting and operating a high-pressure gas discharge lamp, by meansof a switch mode power supply having a lamp circuit, comprising ofarranging an ignition sub-circuit, wherein the ignition sub-circuitcomprises:

at least an ignition capacitor;

a switching element for discharging the capacitor at a desired moment intime having a control electrode;

wherein the control electrode of the switching element is connected to acommutator of the lamp circuit via a control sub-circuit which comprisessubstantially passive elements.

With such a method the ignition probability is increased, so that fewerignition pulses are needed to ignite the lamp. The light output of thelamp is thereby improved and the lifespan extended.

In a preferred embodiment the switching element is a TRIAC. Such aswitching element is generally obtainable commercially and is simple tocontrol.

Further advantages and features of the present invention will beelucidated with reference to the annexed figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit diagram of a switch mode power supply accordingto the present invention in a first preferred embodiment;

FIG. 2 shows a circuit diagram of a switch mode power supply accordingto the present invention in a second preferred embodiment;

FIGS. 3A to 3C show graphs of the voltage progression in time atrespective points in the switch mode power supply of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first preferred embodiment of a switch mode power supply according tothe present invention (FIG. 1) comprises a so-called Half BridgeCommutating Forward (HBCF) as a lamp-circuit 1 for controlling a gasdischarge lamp 2, for instance a metal halogenide lamp with a power of35–70 W. The HBCF circuit 1 comprises the capacitors 3 and 4, which havethe function of buffer capacitor and voltage divider, so that point 5 ofthe circuit carries half the rail voltage or Open Circuit Voltage (OCV),for instance 200 V, and the FETs 6 and 7. The FETs 6 and 7 are madeconducting and non-conducting in a commutating fashion at apredetermined frequency such that point 8 of the circuit carries theOpen Circuit Voltage, for instance 400 V, or 0 V. Coil 10 is present toprovide a stabilized current to the series of lamp 2 and the secondarywinding 12 of transformer 14. The secondary winding 12 filters inco-action with capacitors 16 and 18 the varying component of the loadcurrent provided by coil 10.

The ignition sub-circuit 20 is coupled to the HBCF circuit 1 by thetransformer 14 and comprises a ignition or pulse capacitor 22 which isconnected to the rail voltage on buffer capacitor 3 via the ohmicresistor 24. Ignition capacitor 22 can have a very low impedance, so itis capable of withstanding the high dV/dt that is needed in ignitioncircuits. Ignition capacitor 22 forms part of an electrical resonantcircuit for generating voltage pulses for the purpose of igniting lamp2, with a voltage of about 3.5–5 kV, wherein the primary winding 26 oftransformer 14 is likewise included in the circuit together with diode28 and as the switching element switch 30, for instance a TRIAC. Switch30 has a control electrode and is controlled by a control sub-circuitcomprising DIAC 32 with capacitors 34, 36 and the Zener diode 38, sothat no further control logic is required. So the control sub-circuitcomprises substantially passive elements. The control electrode of theswitch 30 thus is connected to the commutator of the lamp circuit viathe control sub-circuit.

In order to ignite the lamp 2 the ignition or pulse capacitor 22 ischarged via the ohmic resistor 24, wherein switch 30 is in openednon-conducting state. If switch 30 is closed by a small current fromDIAC 32, the resonant circuit is then closed so that the ignitioncapacitor discharges via coil 26, diode 28 and switch 30. The currentthrough coil 26 generates via the transformer 14 a high voltage pulseover coil 12 which is also applied to the lamp 2. This process isrepeated until the lamp is ignited.

The moment at which the ignition pulse takes place is of greatimportance here, since ignition probability is the greatest just afterthe moment of commutation. That is the moment at which the FETs 6 and 7change from conducting state to non-conducting state, or vice versa.Switch 30 is controlled for this purpose by DIAC 32, with a breakovervoltage of about 30 V. The use of a DIAC has the advantage of a low costprice. The capacitive divider formed by capacitors 34 and 36, whichproduces no additional losses such as would occur in the case of aresistive divider, has such a value that the DIAC is triggered if thevoltage swing on point 40 equals the OCV. This circuit is improved byadding a zener diode 38, with a breakover voltage of about 200 V, toavoid false triggering of the DIAC, for instance in the case ofasymmetric buffer capacitor voltage distribution during lamp starting.

A second preferred embodiment (FIG. 2) is an improved version of thefirst preferred embodiment, wherein false triggering is even betterprevented. In the embodiment of FIG. 1 the switch 30 can, afterdischarging of the ignition capacitor 22, remain continuously conductivedue to the current running via resistor 24, which is for instance causedby a false triggering of switch 30. Ignition capacitor 22 is then notcharged for a new ignition pulse. This is prevented by addingsemiconductor element 50, for instance a SIDAC with a breakover voltageof 120 V and a minimal current a few times greater than the minimalcurrent necessary for the switch 30 to remain in conducting state. Byapplying semiconductor element 50 the recombination time available forthe charge carriers in (the semiconductor material of) switch 30 isherein extended, for instance until ignition capacitor 22 has once againbeen charged to 120 V.

Several resistors 54, 56 and 58 are further arranged. The ohmic resistor54 of for instance 47 Ω serves to limit the current on the gate ofswitch 30, resistor 56 of for instance 10 kΩ is arranged to reduce thesusceptibility to failure of the gate of switch 30, and resistor 58 offor instance 1 MΩ serves to reduce possible overshoot of the voltage onthe capacitive divider so as to prevent false triggering of switch 30.

Multiple samples of a practical embodiment of the second preferredembodiment (FIG. 2) have been tested for 4000 hours with the use ofcomponents of production quality, wherein no problems were diagnosed. Ina practical realization, the described second preferred embodiment issuitable for igniting and operating 35 W and 70 W metal halogenidelamps, which are marketed by applicant as type CDM. Buffer capacitors 3,4 then have for instance a value of 68 μF, the ignition capacitor 22 hasa value of for instance 33 nF and the resistor 24 is for instance 47 kΩ.In operating state the voltage over the buffer capacitors has a value inthe order of 480 V.

FIG. 3A shows the time progression of the voltage over ignitioncapacitor 22 Uc22, wherein t is the time and one period is about 5 ms.The voltage increases from 0 V to a maximum value in the order of 480 V.Just after the moment of commutation the capacitor is discharged so thatthe voltage thereover decreases to about 0 V. FIG. 3B shows the voltageU40 on point 40 of circuit 1, wherein the voltage thereon is alternatelyabout 0 V and about 480 V. This voltage controls the voltage U42 onpoint 42 of the circuit 1, wherein each time the voltage on point 40varies a voltage is applied at point 42. Capacitors 34 and 36 have forinstance values in the order of 10 nF and 1.2 nF, so that the voltage onpoint 42 has the time progression as shown in FIG. 3C with a peak valueof about 40 V. At this value the DIAC 32, with a breakover voltage ofabout 30 V, transposes into conducting state so that switch 30 opens theresonant circuit and ignition capacitor 22 discharges and a voltagepulse occurs over lamp 2.

The protection sought for the present invention is not limited to theabove described preferred embodiments thereof, in which manymodifications can be envisaged; this protection is defined by theappended claims.

1. Switch mode power supply for igniting and operating a high-pressuregas discharge lamp, having a lamp circuit and comprising an ignitionsub-circuit, which is coupled via a transformer to the lamp circuit,wherein the ignition sub-circuit comprises: at least an ignitioncapacitor; a switching element for discharging the capacitor at adesired moment in time having control electrode; wherein the controlelectrode of the switching element is connected to a commutator of thelamp circuit via a control sub-circuit which comprises substantiallypassive elements.
 2. Switch mode power supply for igniting and operatinga high-pressure gas discharge lamp as claimed in claim 1, wherein theswitching element is a so-called TRIAC.
 3. Switch mode power supply forigniting and operating a high-pressure gas discharge lamp as claimed inclaim 1, wherein the ignition capacitor and the switching element arecoupled via the primary winding of the transformer to the lamp circuitand form a resonant circuit therewith.
 4. Switch mode power supply forigniting and operating a high-pressure gas discharge lamp as claimed inclaim 1, wherein the switch mode power supply comprises a so-called HalfBridge Commutating Forward circuit.
 5. Switch mode power supply asclaimed in claim 1, wherein the control sub-circuit comprises at leastone DIAC, for instance with a breakover voltage of 30V.
 6. Switch modepower supply as claimed in claim 1, wherein the ignition sub-circuitalso comprises a SIDAC so that the switching element does not remainconductive after the ignition capacitor has been discharged.
 7. Switchmode power supply as claimed in claim 1, wherein the control sub-circuitcomprises at least one resistor for distributing a voltage.
 8. Switchmode power supply for igniting and operating a high-pressure gasdischarge lamp, said switch mode power supply having a lamp circuit andcomprising an ignition sub-circuit, which is coupled via a transformerto the lamp circuit, wherein the ignition sub-circuit comprises: atleast an ignition capacitor; a switching element for discharging thecapacitor at a desired moment in time having control electrode; whereinthe control electrode of the switching element is connected to acommutator of the lamp circuit via a control sub-circuit which comprisessubstantially passive elements; and wherein the switch mode power supplycomprises at least one buffer capacitor for storing a voltage andwherein the ignition sub-circuit comprises at least one resistor whichis connected on one side to the buffer capacitor and on the other sideto the ignition capacitor of the ignition sub-circuit.
 9. Switch modepower supply having a lamp circuit and comprising an ignitionsub-circuit, which is coupled via a transformer to the lamp circuit,wherein the ignition sub-circuit comprises: at least an ignitioncapacitor; a switching element for discharging the capacitor at adesired moment in time having control electrode; and at least twocapacitors which are connected as a capacitive divider to the DIAC;wherein the control electrode of the switching element is connected to acommutator of the lamp circuit via a control sub-circuit which comprisessubstantially passive elements.
 10. Switch mode power supply having alamp circuit and comprising an ignition sub-circuit, which is coupledvia a transformer to the lamp circuit, wherein the ignition sub-circuitcomprises: at least an ignition capacitor; a switching element fordischarging the capacitor at a desired moment in time having controlelectrode; and a Zener diode for triggering the DIAC at a desiredvoltage; wherein the control electrode of the switching element isconnected to a commutator of the lamp circuit via a control sub-circuitwhich comprises substantially passive elements.